The overall goal of my research program is to understand the role of the basal ganglia (BG) and superior colliculus (SC) in target selection for saccadic eye movements. In this application we are focused on the relationship of one output nucleus of the BG, the substantia nigra pars reticulata (SNr) to the SC and their combined role in processing events related to choosing a goal for a saccadic eye movement. A central question in cognitive neuroscience is how visual stimuli are identified as objects for perceptual processing or as goals for movements. Historically, the cortico-BG-SC pathway is considered `passive' in the sense that it receives processed cortical information and through a series of sign changes in nuclei that inhibit their targets, provides a transient, disinhibition to the SC to initiate a saccade. In this application we propose that the BG also play an active role in decisions related to selecting where and when to make a saccade. This proposal has two specific aims; 1) determine whether saccade selection results from SC neuronal activity increasing to a threshold and 2) determine whether the inhibitory output of the SNr influences neuronal processing within the SC and the evolution of saccade choice. During the last award cycle, we demonstrated that selecting a saccade resolved competition between movement goals by biasing stimulus interactions within the SC, as seen in extrastriate cortex. Based on those results and our preliminary data, we propose that saccade selection results from interactions among SC neuronal populations. Further, we propose that the BG input biases these competitive interactions. The current proposal extends our work in important ways. First, the experiments proposed here will build on the growing body of evidence suggesting motor areas play an active role in selection for perception and action. Second, the results will provide necessary and detailed information about the relationship of the SNr to the SC and how this pathway contributes to saccade choices. Furthermore, since saccadic eye movement abnormalities are a prominent symptom in a number of diseases involving the BG and cortico-BG circuits, such as Parkinson's disease, Huntington's disease, obsessive-compulsive disorder, attention deficit disorder and Tourette Syndrome, our results should provide important insights into the mechanisms of symptomology in these disease states. The overall goal of my research program is to understand the role of the basal ganglia and superior colliculus in target selection for saccadic (rapid) eye movements. In this application we focus on understanding the relationship of one output nucleus of the basal ganglia and its target structure, the superior colliculus and their combined role in processing events related to choosing a goal for a saccadic eye movement. Because saccadic eye movement abnormalities are a prominent symptom in a number of diseases involving the basal ganglia and cortico-basal ganglia circuits, such as Parkinson's disease, Huntington's disease, obsessive-compulsive disorder, attention deficit disorder and Tourette Syndrome, we expect our results to provide important insights into the underlying physiological mechanisms of symptoms seen in these disease states.